Review Reports

Round 1

Reviewer 1 Report

Comments and Suggestions for Authors

Introduction: Present the study objectives and their rationale early and more clearly. Better justify why a literature review can contribute to the knowledge about CWs.
Table 1 and Table 2: The contents of these tables could be presented in paragraphs. As they are, the tables' readability is compromised.
Table 3 appears to be a long list of articles lacking scientific logic. I suggest reviewing the presentation format of this table, for example, by the benefits.
The space dedicated to items 3.1, 3.2, and 3.3 must be reduced for a more in-depth analysis of the reviewed content. This change is intended to enhance the depth and quality of the section.
Similar logic applies to item 3.4. The text in this section could be more informative.
The presentation of the results compromises the discussion session. A better presentation of the information in the results section could reveal new elements for discussion. For example, instead of reproducing the findings in the discussion, authors could indicate "how" each finding from their review contributes to "which works" in the literature, or "how" this review can "inspire future research."
Analyzing these elements leads me to suggest a more comprehensive revision of this article.

Author Response

The comments are in the attached document.

Author Response File: Author Response.pdf

Reviewer 2 Report

Comments and Suggestions for Authors

The paper aims to provide a systematic review over the sustainable management of organic waste in constructed wetlands.

The abstract is comprehensive and offers the needed highlights related to the topic of the paper, as well as it points out some of the obtained results. 

The introduction offers the needed knowledge related to the field. I would like to ask the authors to provide a roadmap of the paper at the end of introduction. 

Materials and methods section discusses the choice for Scopus database and PRISMA approach on selecting the papers. The choice for Scopus database is supported by the references provided.  Here, I would like to ask the authors to provide the scheme for PRISMA and to provide references that support the choice of the search words stated in section 2.1.1. and 2.1.2. Please discuss to which extend the words that have not been used for search and could have been used might affect the final results in terms of papers included in dataset. 

For the inclusion and exclusion criteria please bring evidence from the scientific literature or better explain why these criteria were necessary - please do so for each criterion. 

In the results section, please try to move beyond interpreting the figures and discuss why these outcomes have been observed - e.g. why China leads in the area of most published documents. 

Please add a more critical approach to the paper reviewed in section 4.

Please add limitations related to the study, with an accent on the limitations due to the selection of the dataset. 

Author Response

The comments are in the attached document.

Author Response File: Author Response.pdf

Reviewer 3 Report

Comments and Suggestions for Authors

This study reviewed the use of organic residues as substrates in CWs. A large number of papers have been investigated regarding the organic carbon sources in CWs. This paper has certain theoretical basis regarding the recyclability of the substrate. Nevertheless, there are some comments which should be addressed to:

1. It is suggested that the technical approach of this review paper be included in the methodology.
2. It is suggested to provide detailed explanations on how to select various types of organic substances as substrates, and at the same time, indicate the methods for recycling and reusing these resources.
3. Please pay attention to the formatting of the titles of each section. They should be in bold and the font size should be consistent.

Circular economy strategies.

4. Please elaborate on how the circular economy strategy is actually implemented.

Author Response

Comments 1: It is suggested that the technical approach of this review paper be included in the methodology.

Response 1: The technical approach of the review has been incorporated into the Methodology section, detailing the systematic steps, database selection, and analytical framework applied.

Comments 2: It is suggested to provide detailed explanations on how to select various types of organic substances as substrates, and at the same time, indicate the methods for recycling and reusing these resources.

Response 2:  detailed explanation has been added in the Methodology section, clarifying the criteria used to select different organic substrates based on physicochemical properties, local availability, and prior performance in pollutant removal. Additionally, the revised Discussion now includes specific methods for recycling and reusing these residues within circular economy frameworks, highlighting their potential for sustainable material recovery and resource efficiency.

Comments 3: Please pay attention to the formatting of the titles of each section. They should be in bold and the font size should be consistent.

Response 3: The formatting of all section titles has been carefully revised to ensure consistency in font size and style. All main headings are now presented in bold, following a uniform formatting structure throughout the manuscript.

Comments 4: Please elaborate on how the circular economy strategy is actually implemented.

Response 4: We appreciate the reviewer’s comment. The manuscript has been updated to elaborate on how the circular economy strategy is implemented. Specifically, Section 4 now details the practical integration of organic waste valorization within constructed wetlands, emphasizing resource recovery, waste-to-resource loops, and local reuse of biomass as part of circular economy practices.

Reviewer 4 Report

Comments and Suggestions for Authors

Overall, the research topic chosen by the author is actually not novel. There are numerous studies on constructed wetlands or engineered wetlands, and many research institutions have conducted research in this field and published a series of very good papers. It is not very innovative.

1. The abstract mentions that "the removal efficiency is 10-15% higher than that of inorganic substrates", but it does not specify which pollutants they are or under what conditions. It is suggested to supplement specific data sources or typical pollutants (such as NH₄⁺, TP, etc.).
2. You mentioned "only using the Scopus database", although you explained the reason, it may still be questioned that the literature coverage is incomplete.
3. In Figure 1, the horizontal axis represents the year and the name of the vertical axis represents the number of published articles. It is obvious that the author made a mistake in marking.
4. It is suggested to explain exactly what "others" in Figure 4 refer to (such as sludge fermentation liquid, aquatic plants, etc.), otherwise readers will be curious.
5. There is some repetition between the results and the discussion. For instance, the classification in Section 3.4 and the content in Figure 4 overlap somewhat with the discussion sections from 4.1 to 4.5 later on. It can be considered to incorporate some descriptive content into the discussion to make the result section more concise.
6. The current conclusion mainly summarizes the findings. If some suggestions on "what to do next" could be added (such as which substrate is suitable for which water quality and how policies support it), it would be more useful to readers.
7. The language could be smoother. Some sentences are too long. For example, there is a sentence on page 2: "Research and a deep understanding of the components..." This sentence is too long and a bit confusing to read. It is suggested to break it down into short sentences.

Author Response

The answers to each comment can be found in the attached document.

Author Response File: Author Response.pdf

Round 2

Reviewer 1 Report

Comments and Suggestions for Authors

The authors must remove the "open access" filter and re-run their screening and eligibility process on the entire set of retrieved articles. This action can increase the number of articles from the initial 325 (from Scopus) and the final 44. The entire analysis (Results, Discussion) must then be updated based on this new, comprehensive dataset.

The discussion should be expanded to explore the systemic implications related to the journal focus. When writing this section, keep in mind that you must declare how your contributions can contribute to the journal's readers. To identify this contribution, please consider the journal scope. This improvement is mandatory. 

The Results section is largely descriptive, consisting of frequency counts (e.g., Figs 1, 3, 4, 5, 6). The paper would be strengthened if the authors synthesized why these trends are observed. For instance, why do China and India dominate? (The discussion touches on this, but it could be linked in the results). Why is VSFCW (54%) the dominant type for these substrates? A brief analytical comment alongside the descriptive results would elevate the section.

In Section 2.1.1, the authors list a specific set of organic substrates in their search string (e.g., "Coconut shell," "Sugarcane bagasse"). This risks missing other relevant materials. While they also include "Organic substrate," they should briefly justify how this specific list was developed (e.g., from a preliminary scoping search) to assure the reader that other common wastes (e.g., 'food waste', 'manure', 'sludge') were not inadvertently excluded from the analysis.

The recommendations in Section 4.7 are valid but somewhat generic ("need more LCA," "long-term stability," "policy frameworks"). The authors could make these recommendations more targeted. For example, given the 66.7% dominance of biochar, a key recommendation should be "comparative techno-economic analysis of decentralized pyrolysis vs. centralized models" or "investigation into the long-term saturation and disposal pathways for biochar substrates."

Author Response

The responses to each comment are provided in the attached document.

Author Response File: Author Response.pdf

Reviewer 2 Report

Comments and Suggestions for Authors

Thank you for the revised version of the paper. I have no further comments.

Author Response

Okey, thanks.

Reviewer 4 Report

Comments and Suggestions for Authors

none

Author Response

Okey, thanks.

Round 3

Reviewer 1 Report

Comments and Suggestions for Authors

The authors must reframe the paper as a "Review of Open Access Literature," acknowledging the heavy bias this introduces.

It would be beneficial to distinctly separate the discussion of "raw organic waste" vs. "processed organic waste (biochar)" regarding their life cycle impacts and energy costs.

Figure 7 presents comparative removal efficiencies with error bars. The methodology section does not explain how this data was extracted or processed. How were average removal rates calculated across studies with vastly different influent concentrations and hydraulic retention times? What do the error bars represent? (Standard deviation, standard error, range?). Given the sample size is only 44 papers, and split into multiple categories (biochar, agricultural, shell, etc.), some bars in Figure 7 must represent very few data points. The statistical robustness of this comparison needs to be clarified.

The authors should provide clear operational definitions for how they classified a study as "Revalorize" versus "Reuse" to ensure reproducibility.

Figure 2 (Keyword map) is somewhat difficult to read due to the density of the network. Please, improve it. 

The references in the main text are numbered, but the reference list provided in the review file has some formatting inconsistencies (e.g., capitalization of titles). Ensure strict adherence to MDPI styling.

The authors should stick to the specific "R" framework definitions provided in the results section to avoid redundancy.

Author Response

Comments 1: The authors must reframe the paper as a "Review of Open Access Literature," acknowledging the heavy bias this introduces.

Response 1: We appreciate the reviewer’s comment. However, we have decided to retain the original title, as it accurately reflects the scope and objectives of the manuscript. The limitation of using open-access literature and the potential associated bias are explicitly acknowledged in the Methods section (lines 146-150).

Comments 2: It would be beneficial to distinctly separate the discussion of "raw organic waste" vs. "processed organic waste (biochar)" regarding their life cycle impacts and energy costs.

Response 2: We thank the reviewer for this valuable suggestion. In response, we have revised the Discussion section (lines 506–520) to clearly differentiate between raw organic substrates and processed substrates such as biochar, with respect to their life cycle impacts and energy requirements. Specifically, we note that raw organic wastes (e.g., agricultural, forest, and shell residues) generally require minimal processing and therefore involve lower energy inputs, while biochar production entails energy-intensive pyrolysis, increasing its environmental footprint despite its enhanced adsorption capacity and microbial support.

Comments 3: Figure 7 presents comparative removal efficiencies with error bars. The methodology section does not explain how this data was extracted or processed. How were average removal rates calculated across studies with vastly different influent concentrations and hydraulic retention times? What do the error bars represent? (Standard deviation, standard error, range?). Given the sample size is only 44 papers, and split into multiple categories (biochar, agricultural, shell, etc.), some bars in Figure 7 must represent very few data points. The statistical robustness of this comparison needs to be clarified.

Response 3:  The clarification regarding the calculation of average removal rates and the representation of error bars has been addressed in the manuscript. Specifically, the methodology for data extraction, processing, and the calculation of averages across studies with different influent concentrations and hydraulic retention times, as well as the meaning of the error bars (standard deviation), has been added in lines 499–512.

Comments 4: The authors should provide clear operational definitions for how they classified a study as "Revalorize" versus "Reuse" to ensure reproducibility.

Response 4:  A brief clarification regarding the distinction between “Revalorize” and “Reuse” has been added to the manuscript to ensure clarity. Specifically, “Revalorize” refers to transforming agricultural by-products or biochar into cost-effective, functional materials, while “Reuse” refers to their direct utilization without significant modification. This clarification has been included in lines 308–311 of the revised manuscript.

Comments 5: Figure 2 (Keyword map) is somewhat difficult to read due to the density of the network. Please, improve it. 

Response 5: The quality and readability of Figure 2 (Keyword Map) have been improved by adjusting the layout, reducing node overlap, and enhancing label clarity, which allows for easier interpretation of the network.

Comments 6: The references in the main text are numbered, but the reference list provided in the review file has some formatting inconsistencies (e.g., capitalization of titles). Ensure strict adherence to MDPI styling.

Response 6: The reference list has been carefully revised to ensure strict adherence to MDPI formatting guidelines, including capitalization, punctuation, and overall consistency with the numbering in the main text.

Comments 7: The authors should stick to the specific "R" framework definitions provided in the results section to avoid redundancy.

Response 7: We thank the reviewer. The definitions of the “R” framework have been clarified in the Results section to ensure consistency and avoid redundancy.

The English throughout the manuscript has been carefully revised and improved for clarity and readability.